1. Field of the Invention
The present invention relates to a steering-wheel revolution number correction system of all-wheel-drive vehicle. More specifically, it relates to a steering-wheel revolution number correction system of a construction equipment provided with an all-wheel-drive, such as a motor grader and a wheel loader.
2. Description of Related Art
Recently, all-wheel-drive for driving all of front and rear wheels is often provided to a motor grader, etc.
The all-wheel drive drives the rear-wheel by an output of an engine transmitted through a transmission and the output of the engine is transmitted to a hydraulic pump, where right and left front-wheels are driven by a pair of hydraulic motors rotated by delivery oil from the hydraulic pump.
Incidentally, when the vehicle turns, because the turning radius of the front-wheel becomes greater than the turning radius of the rear-wheel on account of inner race difference, the front-wheel has to be rotated faster than the rear-wheel. In a rear-wheel-drive vehicle solely driving the rear wheel thereof, the front-wheel freely rotates in accordance with turning radius. However, in a vehicle having an all-wheel-drive where the front-wheel rotates in synchronization with the rotation of the rear-wheel, the rotation difference of the front and rear wheels caused by the inner race difference during turning movement cannot be completely absorbed, so that the vehicle turns with the front-wheel being braked. Accordingly, unless the front-wheel is rotated faster than the rear-wheel, a so-called “tight-corner” braking phenomenon occurs during turning movement and the vehicle does not turn smoothly.
In order to rotate the hydraulic-motor-driven front-wheel faster than the engine-driven rear-wheel, U.S. Pat. No. 4,186,816 discloses switching a control mode of a hydraulic pump to rotate the front-wheel faster than the rear-wheel by a predetermined ratio (several percent).
Further, U.S. Pat. No. 5,147,010 discloses monitoring and controlling the hydraulic pressure applied to the hydraulic motor to change the revolution number of the hydraulic motor in accordance with slippage of the rear-wheel, thereby rotating the front-wheel faster than the rear-wheel.
However, according to the disclosure of the U.S. Pat. No. 4,186,816, because the front-wheel always rotates faster than the rear-wheel by a predetermined ratio, a problem occurs when the vehicle turns at the same speed (the same revolution number of the rear-wheel) and with different turning radius. Specifically, though the front-wheel has to be rotated faster as the turning radius becomes smaller, since the revolution number of the front-wheel is determined irrespective of the turning radius, the front-wheel does not rotate at a revolution number corresponding to the turning radius, so that the tight-corner braking phenomenon cannot be securely prevented.
Further, the rotation of the front-wheel faster than the rear-wheel by the several percent cannot rotate the front-wheel sufficiently fast when the vehicle turns with a small turning radius.
Because the slippage of the rear-wheel is required in the technique shown in U.S. Pat. No. 5,147,010, the front-wheel does not rotate fast when the vehicle is turned and the rear-wheel does not always slip, so that the tight-corner braking phenomenon cannot be securely prevented.
Further, because only the hydraulic pressure to the hydraulic motor is monitored in this technique, when the ground pressure of the front-wheel is decreased and is likely to be slipped when, for instance, the work is conducted while keeping the blade in contact with the ground, the hydraulic motor may be rotated at a high speed without sufficient hydraulic pressure and the front-wheel may keep on slipping. The energy is wasted in the above condition.
On the other hand, an auxiliary lever for manually controlling the rotation of the hydraulic motor may be provided and operated while turning the vehicle for rotating the front-wheel faster than the rear-wheel. In this arrangement, the front-wheel can be rotated faster than the rear-wheel by a desired ratio during turning movement, so that the vehicle can be rotated smoothly.
However, it is so troublesome to operate the auxiliary lever throughout turning the vehicle in addition to steering operation. Further, it takes considerable skill and is not easy to properly adjust the revolution number of the front-wheel in accordance with the speed of the vehicle and the turning radius.
On the other hand, it is possible to calculate the turning radius by detecting the steering angle of the front-wheel and to rotate the front-wheel faster than the rear-wheel based on the turning radius.
However, in an articulating motor grader having the front-wheel and the rear-wheel attached to independent frames, the respective frames being angle-adjustably connected, because the turning radius on the front-wheel side greatly changes relative to the rear-wheel side not only by the steering operation but by articulating the front frame on the front-wheel side relative to the rear frame on the rear-wheel side, accurate turning radius cannot be obtained only by detecting the steering angle.